In order to improve fuel economy, a fuel cut-off control for stopping fuel supply to an engine during running, and a coasting control for coasting the vehicle by disconnecting the engine from a drive line during running have been employed in recent years. Specifically, the fuel cut-off is carried out to stop fuel supply to the engine given that an accelerator is returned to close an accelerator valve completely, and that an engine speed is higher than an idling speed so that a vehicle speed is higher than a predetermined speed. Under the fuel cut-off control, although the combustion of fuel in the engine is stopped, the engine is still rotating by a torque from the drive wheels. Therefore, a braking torque resulting from a pumping loss of the engine and a friction torque is applied to the drive wheels. Consequently, an engine braking force is applied to the vehicle.
For example, the coasting control is carried out when the accelerator is returned during running to coast the vehicle without rotating the engine passively by bringing the clutch into disengagement disposed between the engine and the drive wheels. Therefore, under the coasting control, the engine braking force will not be applied to the vehicle so that the vehicle is allowed to coast utilizing an inertia energy.
The coasting control may be carried out not only by stopping the fuel supply to the engine but also without stopping the fuel supply to the engine while lowering the engine speed close to the idling speed. In case of carrying out the coasting control while stopping the engine, the fuel will not be consumed during coasting the vehicle so that the fuel economy can be improved significantly. By contrast, in case of carrying out the coasting control without stopping the engine, the fuel economy will not be improved as the case of stopping the engine. In this case, however, it is not necessary to arrange additional devices for insuring a required oil pressure under cessation of the engine such as an electric oil pump, a hydraulic accumulator etc. Therefore, the coasting control can be carried out easily without requiring additional modification of the vehicle and additional equipment. In the following explanation, the coasting control without stopping the engine will be called the “neutral coasting control” or the “N coasting control”.
For example, Japanese Patent Laid-Open No. 2011-163535 describes a controller for mechanical automatic transmission configured to carry out the coasting control. According to the teachings of Japanese Patent Laid-Open No. 2011-163535, an input shaft of a geared transmission is connected to an output shaft of an engine though a friction clutch, and an alteration of engagement state of the clutch and a speed change operation of the transmission are carried out automatically. To this end, a coasting of the vehicle is determined based on a vehicle speed, an execution of a speed change of the transmission, and an accelerator opening. Given that the coasting of the vehicle is continued for a predetermined period of time, the clutch is brought into disengagement and the vehicle speed is lowered to an idling speed. In case the vehicle is not coasting after disengaging the clutch, a target gear stage of the transmission is established according to the vehicle speed and the accelerator opening, and then the clutch is brought into engagement.
Japanese Patent Laid-Open No. 2011-7139 describes a cooling device for a multi-cylinder engine. According to the teachings of Japanese Patent Laid-Open No. 2011-7139, the multi-cylinder engine is allowed to be started while reducing number of activated cylinders when started under cold condition. Such reduced cylinder operation control is continued during idling of the engine.
Japanese Patent Laid-Open No. 2005-188432 describes a control device applied to a multi-cylinder engine comprising a plurality of cylinders, an electronic fuel infector injecting fuel the cylinders, an electronic valve activating mechanism opening and closing an intake valve and an exhaust valve of each cylinder. According to the teachings of Japanese Patent Laid-Open No. 2005-188432, the control device stops fuel injection to some cylinders while closing the intake and exhaust valves thereof, upon expiration of preset time while the engine is idled in the vehicle waiting for a traffic light.
Japanese Patent Laid-Open No. 2005-140076 describes a control device for a vehicle having an automatic transmission provided with a torque convertor having a lockup clutch. According to the teachings of Japanese Patent Laid-Open No. 2005-140076, a fuel cut-off is carried out to stop fuel supply to the engine upon satisfaction of a predetermined condition. If the lockup clutch is in engagement or causing a slip, a timing to commence the fuel cut-off is hastened to be earlier than that of the case in which the lockup clutch is in disengagement. In addition, Japanese Patent Laid-Open No. 2005-140076 describes a control to sequentially stop the fuel supply to the cylinders of the engine after a predetermined delay time from a point at which the lockup clutch has been brought into disengagement.
Japanese Patent Laid-Open No. 8-105334 discloses a control device for a variable displacement engine in which intake valves and exhaust valves of deactivated cylinders are closed within a low load operating range. According to the teachings of Japanese Patent Laid-Open No. 8-105334, displacement mode in which some of the cylinders are deactivated is selected based on a shift position and an engine operating range. Basically, the cylinder deactivation mode is selected upon shifting of the shift position to a neutral position. After shifting the shift position from the drive position to the neutral position, the cylinders activated under the drive position are continuously activated within a predetermined period of time from the point at which the shift position was shifted. In addition, according to the teachings Japanese Patent Laid-Open No. 8-105334, such cylinder deactivation is carried out after raising the idling speed of the engine to prevent an engine stall and to reduce vibrations.
As described, according to the teachings of Japanese Patent Laid-Open No. 2011-163535, the friction clutch is brought into disengagement during the coasting of vehicle so that the vehicle is allowed to be propelled by inertia force, and the engine speed is lowered to the idling speed. That is, the neutral coasting control is carried out to disconnect the engine from the transmission line to idle for improved fuel economy. The engine is operated at the idling speed even under the neutral coasting so that an oil pump or other auxiliary mechanism can be driven by the engine. For this reason, the neutral coasting control taught by Japanese Patent Laid-Open No. 2011-163535 may be adapted for the conventional vehicles. However, the engine is kept driven under during the neutral coasting control although not involved in propulsion of the vehicle. Consequently, more fuel is consumed in comparison with the neutral coasting while stopping the engine. Thus, the coasting neutral control has to be improved to improve fuel economy.